Name: x-linked inheritance in families
Uploaded: 2025-10-23T11:49:32.000Z
Duration: 1 min 19 s
Channel: JoVE

K12 Standards

Virginia  Standards of Learning

HIgh School

Life Science

LS.10

LS.10.c

X-linked Traits

- [Istruttore] Dato che uomini e donne

ereditano un numero
diverso di cromosomi X,

i tratti codificati dal cromosoma X

mostrano diversi modelli
di espressione tra i sessi.

Tratti codificati da geni sul cromosoma X

si definiscono geni legati al cromosoma X.

Poiché le femmine hanno
due copie del cromosoma X

i maschi solo una, una mutazione recessiva

su un gene legato all'X

si manifesterà con più
frequenza nei maschi

che nelle femmine,

chi possiede una copia sana del gene

sull'altro cromosoma X.

Un esempio di una
caratteristica legata all'X

che si manifesta nei maschi,

più spesso che nelle
femmine è il daltonismo,

in cui la forma più comune

è dovuta a mutazioni nei
geni che codificano il rosso

e recettori fotografici del
colore verde sul cromosoma X.

Con due cromosomi X e due
copie di ciascun gene,

le femmine possono
compensare queste mutazioni

se portano copie sane degli stessi geni

su uno dei due cromosomi X.

Tuttavia, senza una controparte
non mutata, una mutazione

lascerà i maschi

senza una copia funzionante
e una limita sensibilità

ai colori

rendendo difficile alle
persone la distinzione

dei colori rosso, giallo e verde.

Nienhuis, Arthur W., Amit C. Nathwani e Andrew M. Davidoff. "Terapia genica per l'emofilia". Terapia molecolare 25, n. 5 (2017): 1163-1167. [<a href="https://doi.org/10.1016/j.ymthe.2017.03.033">Sorgente</a>]

tratti associati al cromosoma x

Nella maggior parte delle specie di mammiferi, le femmine hanno due cromosomi sessuali X e i maschi hanno una X e una Y. Di conseguenza, le mutazioni sul cromosoma X nelle femmine possono essere mascherate dalla presenza di un normale allele sulla seconda X. Al contrario, una mutazione sul cromosoma X nei maschi causa più spesso difetti biologici osservabili, in quanto non esiste una X normale per compensare. Le variazioni del tratto derivanti da mutazioni sul cromosoma X sono chiamate "X-linked".

Un esempio ben studiato di un tratto legato a X è la daltonismo. Quando si verifica una mutazione nei geni responsabili della visione di colore rosso e verde nei fotorecettori della retina, può verificarsi la mancanza di visione del colore. Anche se questa mutazione recessiva può far sì che le femmine siano daltoniche, devono possedere due cromosomi X mutati. La cecità del colore è molto più comune nei maschi, che hanno solo un cromosoma X e quindi nessuna seconda copia per compensare potenzialmente la mutazione.

Il daltonismo è passato di madre in figlio; una X mutata sarà passata dalla madre alla metà dei suoi figli, che riceveranno una Y dal padre. Nel frattempo, i maschi daltonici passeranno sull'allele per la daltonismo a tutte le loro figlie, che saranno portatori o daltonici, a seconda dell'allele materno ricevuto. Occasionalmente, questo tipo di mutazione X-linked può anche sorgere per mutazione spontanea e non come risultato dell'eredità dalla madre o dal padre.

Un altro esempio ben studiato di una condizione legata all'X è l'emofilia, un disturbo emorragico ben noto per il suo alto tasso di incidenza nelle monarchie europee. L'emofilia è il risultato di una mutazione in un fattore di coagulazione del sangue, VIII o IV, che porta a sanguinamento più lungo dopo l'infortunio e può causare sanguinamento articolare spontaneo. Anche se attualmente non esiste una cura per l'emofilia, la terapia genica viene esplorata come una possibilità.

Tratti associati al cromosoma X

Nella maggior parte delle specie di mammiferi, le femmine hanno due cromosomi sessuali X e i maschi hanno una X e una Y. Di conseguenza, le mutazioni sul cromosoma X nelle femmine possono essere ...

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Genetica classica e moderna

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

x-linked inheritance in families

X-linked inheritance explains how traits on the X chromosome are passed through families. In most mammalian species, females have two X sex chromosomes, while males have one X and one Y. A mutation on one X chromosome may be masked in females by a normal allele on the other X. In males, that same mutation is more likely to show because there is no second normal X to balance it.

Traits caused by changes on the X chromosome are called X-linked traits. Color blindness is a well-studied example. It can happen when mutations affect the genes for red and green color vision in the photoreceptors of the retina. This recessive mutation can cause color blindness in females, but they usually need two mutated X chromosomes for the trait to appear.

Color blindness is much more common in males because they have only one X chromosome. The trait is often passed from mother to son. A mother can pass a mutated X chromosome to half of her sons, and sons receive a Y chromosome from their father. A colorblind male will pass the color blindness allele to all of his daughters. Those daughters may be carriers or color blind, depending on the allele they receive from their mother.

Sometimes an X-linked mutation can also appear through spontaneous mutation instead of inheritance from a parent. Another well-known X-linked condition is hemophilia, a bleeding disorder that has appeared at high rates in European monarchies. Hemophilia results from a mutation in a blood clotting factor, either VIII or IV. It can cause longer bleeding after injury and spontaneous joint bleeding. There is no cure yet, but gene therapy is being explored as a possible treatment.

X-Linked Inheritance in Families

genetic lingo

Genotype, Phenotype, and Inheritance

Genotype, phenotype, and inheritance help explain how traits and disease risk are passed from parents to offspring. In diploid organisms, including humans, most other animals, and many plants, somatic cells carry a duplicate set of chromosomes. These chromosome pairs are homologous, with similar length, centromere position, and gene locations. Diploid organisms inherit two versions of each gene, one from each parent. These versions are called alleles, and they sit at the same relative locus,...

punnett squares

Punnett Squares for Predicting Traits

Punnett squares help predict which traits offspring may inherit from two parents. They show possible genotypes, which are pairs of alleles, and can also show the chance of a phenotype, or visible trait, when the inheritance pattern is known. These grids work best for traits controlled by a single gene locus and passed on independently during meiosis. Punnett squares were developed in the early 1900s by Reginald Punnett. His work came after Gregor Mendel’s pea plant experiments revealed the...

monohybrid crosses

Mendel’s Pea Plant Inheritance Ratios

Gregor Mendel used pea plants to study how traits are passed from parents to offspring. His breeding experiments in the 1850s and 1860s showed that inheritance follows clear patterns. He crossed true-breeding plants that differed in one trait and found that dominant traits can hide recessive traits. Mendel focused on seven pea plant traits, and each trait had two forms controlled by a single gene locus. He noticed that true-breeding plants always produced offspring with the same phenotype, or...

dihybrid crosses

Mendel's Law of Independent Assortment

Mendel’s law of independent assortment explains how two traits can be inherited separately. Gregor Mendel tested this idea with pea plants that differed in two characteristics, such as pea color and pea shape. He used true-breeding parents that were homozygous for both traits but showed different phenotypes. In one example, Mendel crossed round, yellow peas with wrinkled, green peas. The parental generation had RRYY and rryy genotypes. Their first-generation offspring were all dihybrids,...

law of segregation

Mendel’s Pea Cross Ratios

Mendel’s pea plant crosses show how traits can disappear in one generation and return in the next. When he crossed plants, one parental trait sometimes seemed to vanish in the F1 generation and reappear in the F2 generation. Mendel used this pattern to explain why some traits are dominant and others are recessive. He proposed that traits are controlled by alleles, which are different versions of a gene. An organism inherits two copies of each gene, one from each parent. In his model, a...

law of independent assortment

Mendel’s Dihybrid Cross and Trait Ratios

Mendel’s dihybrid cross shows how two traits are inherited at the same time. A dihybrid cross is a cross in which both parents are homozygous for two distinct traits, and the F1 generation is heterozygous for both traits. Mendel used pea plants with round yellow peas and wrinkled green peas as an example. One parent had genotype YYRR, and the other had genotype yyrr. In the F1 generation, every plant showed the dominant traits, so the plants were YyRr and had yellow, round peas. The F2...

test cross

Using Test Crosses to Identify Genotypes

Test crosses are used to identify the genotype of an organism with a dominant trait. A genotype is the pair of alleles inherited for a gene, and alleles are different forms of the same gene. In diploid organisms, including humans, each gene usually has two alleles, one from each parent. An allele is recessive when its effect is hidden by another allele at the same gene location. Pea plants provide a clear example with flower color. White flowers are recessive, because one copy of the purple...

probability laws

Inheritance Risk with Sum and Product Rules

Inheritance risk can be calculated with the sum rule and the product rule. The sum rule applies to mutually exclusive events, or events that cannot happen at the same time. The product rule applies to independent events, or events that do not affect one another. These rules describe theoretical probability, which is the chance of an event before it happens. Empirical probability is different because it is based on events that have already occurred. These probability rules are useful in...

pedigree analysis

Reading Family Traits in Pedigrees

Pedigrees are family trees used to track how a trait passes through generations. They help scientists and students see whether a trait is dominant or recessive, whether it is autosomal or sex-linked, what genotypes family members may have, and what phenotypes may appear in future generations. For families with a history of inherited disease, this information can be important for family planning. In plants and animals, inheritance is often studied with crosses. Monohybrid crosses can show...

multiple allele traits

Blood Type and Allele Interactions

Multiple alleles can shape traits through blood type, sickle cell anemia, incomplete dominance, and codominance. Multiple allelism means a gene has three or more allelic forms. Diploid organisms, including humans, usually carry only two alleles for each gene, but many human genes exist in multiple forms in the population. Human blood type is a clear example of multiple allelism. The blood type gene has three alleles: I A , I B , and i. These alleles help determine which blood type a person has...

epistasis

Gene Masking in Rabbit Fur Color

Epistasis describes how one gene can mask or change the effect of another gene. In high school genetics, this helps explain why many traits do not follow a simple single-gene pattern. Instead, alleles at different locations can work together to shape the final phenotype. Mendel studied seven unrelated pea traits, but most traits involve multiple gene interactions. Epistasis often happens when one gene interferes with the expression of another gene. It can also occur when genes are part of the...

polygenic traits

Polygenic Traits in Human Height

Polygenic traits are traits controlled by more than one gene. Human height is a clear example of this kind of inheritance. Because many genes work together, the final trait can vary a lot from person to person. Researchers have found hundreds of loci, or gene locations, that influence height. There are likely many more that have not yet been identified. Many of these genes affect cartilage in the growth plates of the long bones in the arms and legs, either directly or indirectly. Height is...

pleiotropy

SOX10 and Multiple Traits in Pleiotropy

Pleiotropy is when one gene affects more than one trait. The SOX10 gene is a clear example because it can influence pigmentation, hearing, and intestinal movement. These traits may seem unrelated, but they can all be linked to the same gene change. In humans, a deleted region in SOX10 can cause Waardenburg Syndrome Type 4, or WS4. WS4 is an autosomal recessive disorder, which means a person usually inherits two altered copies of the gene. Common signs include a white forelock,...

nature and nurture

Genes and Environment Shape Height

Genes and environment both shape human height and other traits. Genes are the inherited instructions we get from our parents. Environment includes factors such as diet, stress, and social conditions. Many phenotypes, or observable traits, reflect both influences. Some also show up later in life, including cancer and other diseases. Height is a clear example of this interaction. Scientists estimate that 70-90% of height variation comes from genetic differences among people. The remaining 10-30%...

chromosomal theory of inheritance

How Genes Map to Chromosomes

Genes map to chromosomes, and this idea explains how Mendel’s inheritance patterns work. Gregor Mendel published his pea plant breeding results in 1866. He showed that physical traits follow predictable patterns. At the time, scientists did not yet know that genes existed. Mendel used the word "factors" for the hereditary units he studied. The details behind Mendel’s laws of segregation and independent assortment were still unknown for many years. In the late 1800s, better microscopes and...

non-nuclear inheritance

Maternal DNA in Mitochondria and Chloroplasts

Maternal DNA in mitochondria and chloroplasts explains how some genes are inherited outside the nucleus. Most DNA is in the nucleus, but mitochondria and chloroplasts in the cell cytoplasm also carry their own DNA and can copy it independently of nuclear DNA. Mitochondria are found in both plant and animal cells. They are called the powerhouses of eukaryotic cells because they break down glucose to make energy for cellular activity. Mitochondrial DNA has about 37 genes, and many help with...

x-linked traits

X-linked inheritance explains how traits on the X chromosome are passed through families. In most mammalian species, females have two X sex chromosomes, while males have one X and one Y. A mutation on one X chromosome may be masked in females by a normal allele on the other X. In males, that same mutation is more likely to show because there is no second normal X to balance it. Traits caused by changes on the X chromosome are called X-linked traits. Color blindness is a well-studied example.

sex-linked disorders

X- and Y-Linked Disorder Patterns

X- and Y-linked disorders come from mutations in genes on the sex chromosomes. These genes, like those on autosomes, are needed for normal body function. When a mutation causes a biological problem, the condition is called sex-linked. Y-linked disorders come from changes in the Y chromosome. They affect only males, because only males carry a Y chromosome. Mutations in this small chromosome can affect male sexual function and secondary sex characteristics. One example is Y-chromosome...

x-inactivation

Barr Bodies and X-Chromosome Balance

Barr bodies and X-chromosome balance help explain how female mammalian cells control gene activity from the X chromosome. The human X chromosome carries far more genes than the Y chromosome. Because males usually have one X chromosome and females have two, females would make twice as many of some proteins without a control system. To prevent that imbalance, female mammalian cells inactivate nearly all the genes on one X chromosome during early embryonic development. The inactive X condenses...

key terms and concepts

Learn how X-linked traits are inherited, why color blindness is more common in males, and how hemophilia affects blood clotting.

X-Linked Inheritance in Families | JoVE K12

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Next Generation Science Standards

High School

heredity-inheritance-and-variation-of-traits

Heredity: Inheritance and Variation of Traits

HS-LS3-1

new-york-state-science-learning-standards

New York State Science Learning Standards

Inheritance and Variation of Traits

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X-Linked Inheritance in Families

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